There are a number of television broadcast standards used around the world which are not compatible with one another. For example, Europe uses “Digital Video Broadcasting” (DVB) standards, while North America uses “Advanced Television Systems Committee” (ATSC). Furthermore, Japan uses “integrated Services Digital Broadcasting” (ISDB) while China uses its homegrown “Digital Multimedia Broadcast Terrestrial/Handheld” (DMB-TH) standard. In addition there are cable, satellite, handheld and terrestrial TV standards.
Television is facing competition from the internet one at least two fronts. For example users can buy or rent TV series and films on a number of websites and watch them without advertisements. Users may also download the content from sites like Hulu.com using streaming technology to receive the content with advertisements. Other subscription based businesses are being developed to allow users to access the content, for example, in exchange for a monthly payment. A second front of competition with the internet is that of time. For example, many people spend time using applications like social networks, search engines, instant messaging, e-mails, voice-over IP, games, etc. As the users spend more time on the internet they spend less time watching television.
Different standards have been developed for digital television. For example, the MPEG-2 standard defined by the Motion Picture Expert Group (MPEG) is a standard that allows television to convert their analog systems into more efficient digital television systems. There are other MPEG industry standards. For example MPEG4 offers a more efficient video compression. A Standard Definition (SD) television requires approximately 3.8 Mbps in MPEG-2 and 1.8 Mbps in MPEG-4. A High Definition (HD) television requires approximately 19 Mbps in MPEG-2 and around 7 Mbps in MPEG-4.
The MPEG-2 standard specifies formatting for the various component parts of a multimedia program. Such a program might include, for example, MPEG-2 compressed video, compressed audio, control data and/or user data. The standard also defines how these component parts are combined into a single bit stream. The process of combining the components into a single stream is known as multiplexing. The multiplexed stream may be transmitted over any of a variety of links, such as Radio Frequency Links (UHF/VHF), Digital Broadcast Satellite Links, Cable TV Networks, Standard Terrestrial Communication Links, Microwave Line of Sight (LoS) Links (wireless), Digital Subscriber Links (ADSL family), Packet/Cell Links (ATM, IP, IPv6, Ethernet.)
To compress a stream carrying multimedia entertainment content, discrete samples in a stream are transformed into a bit-stream of tokens, which uses less bandwidth than the corresponding initial stream, since essentially only data that has changed from image to image is captured in the compressed stream, instead of capturing all the information from each image. The signal is broken into convenient sized data blocks (frames, or packets), and header information is added to each data block. The header typically identifies the start of the packets and may include time-stamps. The multimedia encoding/decoding format tells the decoder (receiver) how to inverse-represent the compacted stream back into data resembling the original stream of un-transformed data, so that the data may be heard and viewed in its normal form.
MPEG systems are composed of various types of streams, such as, for example, Elementary Streams (ES), Packet Elementary Streams (PES), Program Streams (PS) and Transport Streams (TS). Elementary Streams (ES) contain the raw information components stream of a program stream, for example the compressed information of an audio stream of a program or the compressed information of a video stream of a program. Elementary streams in MPEG are first packetized in variable-length packets called PES packets which primarily have a length of 64 kbytes and begin with a PES header of 6 bytes minimum length. A Packet Elementary Stream (PES) is a raw information component stream that has been converted to packet form, such as, for example, a sequence of packets. This packetization process involves dividing a group of bits in an elementary stream and adding packet header information to the data. The packet header includes a Packet Identification code (PID) that uniquely identifies the packetized elementary stream from all other packetized elementary streams that may be transmitted. This Packetized Elementary Stream (PES) with its relatively long packets structures is not optimal for broadcasting transmission.
The MPEG-2 standard defines two forms of multiplexing (combining of ES into a single stream): MPEG Program Streams (PS) and MPEG Transport Streams (TS).
A MPEG Program Stream contains a group of tightly coupled PES packets referenced to a common time base like, for example, a television program. Such streams are suited for transmission in a relatively error-free environment and enable easy software processing of the received data.
In MPEG Transport Streams, each PES packet is broken into fixed-sized transport packets, providing the basis of a general-purpose technique for combining one or more streams, possibly with independent time bases. This is suited for transmission in which there may be potential packet loss or corruption by noise, and/or where there is a need to send more than one program at a time. In MPEG-2, the objective has been to assemble up to 20 independent TV or radio programs to form one common multiplexed MPEG-2 data signal.
The MPEG Transport Stream consists of a sequence of fixed sized transport packets of 188 bytes. Each packet comprises 184 bytes of payload and a 4 byte header. One of the items in this 4 byte header is the 13 bit Packet Identifier (PID).
MPEG-2 Transport stream (TS) is a standard format for transmission and storage of audio, video, and data, and is used in broadcast systems such as DVB and ATSC. Transport Stream is specified in MPEG-2 Part 1, Systems (formally known as ISO/IEC standard 13818-1 or ITU-T Rec. H.222.0).
The first header byte of a TS packet is the “sync byte,” whose value is 0x47, followed by three one-bit flags and a 13-bit Packet Identifier (PID). This is followed by a 4-bit continuity counter. Additional optional transport fields, as signaled in the optional adaptation field, may follow. The rest of the packet typically consists of payload. Packets are 188 bytes in length, but the communication medium may add some error correction bytes to the packet. ISDB-T and DVB-T/C/S uses 204 bytes and ATSC 8-VSB, 208 bytes as the size of emission packets (transport stream packet+FEC data). ATSC transmission adds 20 bytes of Reed-Solomon forward error correction to create a packet that is 208 bytes long. The 188-byte packet size was originally chosen for compatibility with ATM systems.
A Transport Stream specifies a container format encapsulating packetized Elementary Streams, with error correction and stream synchronization features for maintaining transmission integrity when the signal is degraded. Transport Stream transmissions may carry multiple Program Streams.
An Elementary Stream in a Transport Stream is identified by a 13-bit packet identifier called PID. A demultiplexer extracts Elementary Streams from the Transport Stream in part by looking for packets identified by the same PID. Packets in the same Elementary Stream have the same PID, so that the decoder can select the Elementary Streams it wants and reject the remainder. Currently, the elementary video, audio and data streams for the same channel use a different PID.
A Transport Stream may include Electronic Program Guide (EPG) information and Program Specific Information (PSI), which describe the Elementary Streams which need to be combined to build programs.
Broadcast systems, like for example DVB, do not only transmit pure content, but also descriptions about the content in the form of metadata. This metadata contains different kind of content-information and may be use to navigate through the content, for example to select different television channels. For example, in MPEG-2 the metadata may be transmitted using the Program Specific Information (PSI) packets.
As discussed above, Program Specific Information is the MPEG-2 data that identifies what parts of the transport stream belong to a particular program. This information is carried in a number of PSI tables:                Program Association Table (PAT)        Program Map Table (PMT)        Conditional Access Table (CAT)        Network Information Table (NIT)        
FIG. 1 is a diagram showing DVB MPEG-2 elementary streams, including audio streams, video streams, data streams and the associated Program Map Tables (PMT) and Program Association Table (PAT).
The Program Association Table (PAT) is the entry point for the Program Specific Information (PSI) tables. It lists all programs available in the transport stream. It is carried in packets with PID=0. For each assigned program number, the PAT lists the PID for packets containing that program's PMT.
The PAT includes data that the decoder uses to determine which programs (also referred to as channels) exist in the respective transport stream. Each of the listed programs is defined by a 16-bit value called program number. Each of the programs listed in PAT has an associated value of PID for its Program Map Table (PMT). The PAT points to a number of PMTs (one per program), which, in turn points to the video, audio, and data content of a respective program carried by the stream.
Program Map Tables (PMTs) contain information about programs. The Program Map Table (PMT) lists all the PIDs for packets containing elements of a particular program (for example, audio, video, and auxiliary data). For each program, there is one PMT. Once the PIDs for the video, audio and data content of the respective program carried by the stream are known, the decoder is able to decode the packets that have these PIDs.
While the MPEG-2 standard permits more than one PMT section to be transmitted with a single PID, most MPEG-2 television systems such as ATSC and SCTE require each PMT to be transmitted with a separate PID that is not used for any other packets. The PMTs provide information on each program present in the transport stream, including the program number, and list the elementary streams that comprise the described MPEG-2 program. There are also locations for optional descriptors that describe the entire MPEG-2 program, as well as an optional descriptor for each elementary stream. Each elementary stream is labelled with a stream type value.
The MPEG transport decoder generally performs the following functions:                1. read the PAT to find the PMT for a desired program,        2. demultiplex the packets that carry the desired PMT        3. read the PMT        4. demultiplex the packets (with PIDs specified in the PMT) into the various elemental streams        
The MPEG-2 specification does not specify the format of the CAT and NIT.
A CAT is used for a scrambled stream. The CAT is carried in packets with PID=1. The CAT contains PIDs for Entitlement Management Messages (EMMs), which contain authorization level information for conditional access systems.
To cope with any extensions, the MPEG Group has created the possibility to incorporate so-called “private sections and private tables” in the transport stream. The group has defined mechanisms which specify what a section of a table has to look like, what its structure has to be and by what rules it is to be linked into the transport stream. Taking advantage of the “private section” and “private tables” features, the European DVB Group has introduced numerous additional tables intended to simplify the operation of DVB receivers. Called “Service Information” (SI) they are defined in ETSI Standard ETS300468. Some of these tables are the “Network Information Table”, the “Time&Date Table” (TDT), and the “Time Offset Table” (TOT).
The Network Information Table (NIT) is an optional table that describes all physical parameters of a DVB transmission channel. It contains, for example, the received frequency and the type of transmission (e.g. satellite, cable, terrestrial) and also the technical data of transmission like error protection, type of modulation, etc. This table may be used to optimize the channel scan as much as possible. FIG. 1 shows an example of a Program Association Table (PAT) containing the PID for a Network Information Table (NIT).
In Europe, many broadcasters are also transmitting an “Electronic Program Guide” (EPG) which has its own table in DVB, the so-called “Event Information Table” (EIT). It contains the planned starting and stopping times for the broadcasts of, e.g. one day or one week. The structure which is possible here is very flexible and also allows additional information to be transmitted.
The “Time&Date Table” (TDT) is used to transmit the current clock time and the current date. In the TDT, Greenwich Mean Time (GMT), i.e. the current clock time for the Zero-Degree meridian without any daylight saving time shift is transmitted. The respective applicable time offset can then be broadcast in a “Time Offset Table” (TOT) for the various time zones. It depends on the software of the TV receiver how the information contained in the TDT and TOT is evaluated. Complete support for this broadcast time information may require the DVB receiver to be informed of its current location in a country having a number of time zones.